1. Field of the Invention
The present invention relates to a technique of forming a channel stop doping region on a silicon substrate disposed beneath a field oxide film in a semiconductor device, and more particularly to an isolation structure of a semiconductor device capable of selectively increasing a channel stop doping concentration only at a portion of a silicon substrate disposed beneath an edge of a field oxide film and thereby improving an insulating characteristic of a field transistor with a small pattern, and to a method for forming such an isolation structure.
2. Description of the Prior Art
In integrated circuit field, generally, as means for insulating active regions of a silicon substrate with one another, a local oxidation of silicon (LOCOS) process has been mainly used to form a field oxide film on a field region of the silicon substrate.
In accordance with the LOCOS process, a pad oxide film is formed over a single crystalline silicon substrate. A nitride film pattern is then formed only on a portion of the pad oxide film disposed over an active region of the single crystalline substrate. Using the nitride film pattern as a mask, a field oxide film is selectively formed on a field region of the single crystalline substrate.
In a case of an integrated circuit fabricated using the LOCOS process, a field transistor, that is, an N-field transistor is parasitically formed between adjacent active regions of n.sup.+ diffusion layers formed on a p type single crystalline silicon substrate.
This will be described in more detail. In the integrated circuit to which the LOCOS process is applied, a pad oxide film and a nitride film are sequentially formed over the p type single crystalline silicon substrate. The nitride film is then patterned so that it remains only at its portions respectively corresponding to active regions of n.sup.+ diffusion layers on the silicon substrate. Using the patterned nitride film as a mask, p type impurity ions such as boron ions are implanted as a channel stop doping ion in a field region of the silicon substrate. A field oxide film is then selectively formed on a field region of the silicon substrate using a self alignment doping process. As a result, a parasitic N-field transistor is formed which is constituted by active regions of n.sup.+ diffusion layers and the ion-implanted channel stop region disposed between the active regions.
In this case, however, a segregation phenomenon that boron ions implanted in the silicon substrate move toward the field oxide film occurs during the formation of the field oxide film. Due to such a segregation phenomenon, the concentration of boron ion at the interface between the field oxide film and the silicon substrate is decreased after completion of the formation of field oxide film. This results in a decrease in threshold voltage of the parasitic field transistor.
In the integrated circuit to which the LOCOS process is applied, a bird's beak phenomenon also occurs at a portion of the field oxide film disposed in a boundary region between the field region and each active region. Such a bird's beak of the field oxide film penetrates the active region, thereby causing the active region to be substantially reduced.
Due to the lateral diffusion of channel stop ions occurring during the formation of field oxide film, the active region is substantially reduced, thereby resulting in an increase in the junction capacitance between the active region and the corresponding diffusion layer. This also results in an increase in junction leakage current. As a result, there is a limitation on high integration of semiconductor devices.
In order to effectively achieve high integration of semiconductor devices, there have been newly proposed various methods for minimizing the bird's beak and improving the channel stop doping.
One of such methods is to avoid the segregation of channel stop ions at the interface between the field oxide film and the single crystalline silicon substrate. As such a method, a through field ion implantation method has been used which is illustrated in FIG. 1. In accordance with the through field ion implantation method, as shown in FIG. 1, a pad oxide film 2 is formed over a single crystalline silicon substrate 1. In the silicon substrate 1, a n.sup.+ diffusion regions 3 is formed at each active region. A field oxide film 4 is formed on each field region of the silicon substrate 1. In the entire structure of the silicon substrate 1 with the field oxide film 4, boron ions 5 for channel stop doping are subsequently implanted at a high energy. By virtue of the structure, the implanted boron ions 5 is prevented from being segregated.
In the structure obtained in accordance with the through field ion implantation method, however, the threshold voltage of field transistor varies sensitively depending on the thickness of field oxide film 4 that is dependent upon the size of a pattern for isolation region. Even when the same oxidation condition is used in field oxide films having different isolation region pattern sizes, the field oxide film of a smaller pattern size has a smaller thickness than the field oxide film of a larger pattern size. This may be because a concentration of stress occurs at an edge of the isolation region pattern.
For this reason, the penetration depth of boron ions in the silicon substrate 1 is larger at a region where the field oxide film 4 has a smaller thickness than at a region where the field oxide film 4 has a larger thickness. As a result, it is difficult to complement the concentration of channel stop ions at the interface between the field oxide film and the silicon substrate in accordance with the through field ion implantation method.
Where a blank ion implantation is used in combination with the through field ion implantation method, an increase in ion concentration occurs at a portion of the silicon substrate disposed beneath the junction of active region. This results in an increase in junction capacitance and a decrease in the junction breakdown voltage of the n+/p junction. When the ion implantation is carried out under a condition that a mask is defined using a photoetch process, in order to prevent the above-mentioned problem, only a portion of the p type silicon substrate corresponding to the field region may have an increased ion doping concentration selectively. In this case, however, an alignment tolerance is present between the mask and the field region. Consequently, this method is difficult to be applied to a structure having field regions with a small pattern size.
In order to solve this problem, there has been proposed a new through field ion implantation method using a channeling. A structure fabricated in accordance with this method is illustrated in FIG. 2. As shown in FIG. 2, in each active region of a p type single crystalline silicon substrate 11, a n.sup.+ diffusion regions 13 is formed. A field oxide film 14 is formed on each field region of the silicon substrate In the entire structure of the silicon substrate 11 with the field oxide film 14, boron ions 15 for channel stop doping are subsequently implanted.
In accordance with the method of FIG. 2, the ion implantation is carried out under a condition that an ion beam alignment has been adjusted, thereby increasing the ion concentration at the interface between the field oxide film and the silicon substrate in either of the region where a field oxide film with a small pattern is disposed or the region where a thin field oxide film with a large pattern. In accordance with this method, a channeling is induced in the active region. As a result, it is possible to improve the junction capacitance and the junction breakdown voltage.
However, this method also involves a variation in the penetration depth of channel stop ions depending on the pattern size of the field region. As a result, a variation in insulating characteristic may occur.
The method which fabricates the structure of FIG. 2 will be described in detail in conjunction with FIGS. 3A to 3C. In FIGS. 3A to 3C, elements respectively corresponding to those in FIG. 2 are denoted by the same reference numerals.
In accordance with the method, first, over a single crystalline silicon substrate 11, a pad oxide film 12 is formed to a thickness of about 350.ANG., as shown in FIG. 3A. A nitride film 16 is then formed to a thickness of 1,500.ANG. over the pad oxide film 12. Thereafter, the nitride film 16 and the pad oxide film 12 are partially removed at their portions respectively disposed over field regions, having different pattern sizes, of the silicon substrate 11 using the well-known photoetch process, thereby exposing corresponding surface portions of the silicon substrate 110 As a result, patterns of the nitride film 16 and the pad oxide film 12 are obtained which cover active regions of the silicon substrate 11.
Thereafter, the exposed portions of substrate 11 corresponding to the field regions are subjected to a thermal oxidation using the patterned nitride film 16 as an oxidation barrier so as to form a field oxide film 14, as shown in FIG. 3B. The thermal oxidation is carried out such that the field oxide film 14 has a bulk thickness of about 7,000.ANG. at its portion disposed over each field region having a larger pattern size and a maximum thickness of about 4,200.ANG. at its portion disposed over each field region having a smaller pattern size.
The nitride film 16 and the pad oxide film 12 are then completely removed, as shown in FIG. 3C. Subsequently, boron ions are implanted as channel stop ions in the entire surface of the substrate 11 without using any mask. The ion implantation is carried out at an energy of 140 KeV and in a dose of 3.times.10.sup.12 ions/cm.sup.2 under a condition that beams of boron ions are oriented in perpendicular to the surface of substrate 11 oriented in the &lt;100&gt; direction.
In this case, the projected range of boron ions is 4,000.ANG. in the region corresponding to the field oxide film 14 and 8,000.ANG. in the region corresponding each exposed portion of substrate 11.
In other words, in the region where the field oxide film 14 has a minimum thickness the thickness of the field oxide film portion indicated by "a" in FIG. 3 corresponds to the projected range of boron ions. In the region where the field oxide film 14 has a larger thickness, the thickness of the field oxide film portion, namely, a bird's beak portion indicated by "b" in FIG. 3 corresponds to the projected range of boron ions.
As apparent from the above description, all the conventional methods involve a variation in the thickness of field oxide film depending on the pattern size of isolation region. As a result, they can not obtain a uniform insulating characteristic because of a variation in the concentration of boron ions for channel stop at the interface between the field oxide film and the silicon substrate and a variation in interface occupying area both caused by the variation in the thickness of field oxide film. Moreover, the boron ions are implanted even in the active region. This results in an increase in junction capacitance and an increase in leakage current.